The statement, issued by the Ministry of Foreign Affairs, tells citizens to "exercise appropriate caution", especially in cities affected by "tensions... over shootings of young black males by police officers".

"In particular young males are asked to exercise extreme caution in affected cities in their interactions with the police. Do not be confrontational, and co-operate," it says.

"Do not get involved in political or other demonstrations under any circumstances and avoid crowds."

The US regularly issues travel advisories for Americans visiting other countries, but it is rare for nations to issue warnings for their citizens travelling to the US.